The liver is darker than the kidneys possibly because of the higher content in deoxygenated blood, bile pigment, and lipofuscin (discussed in the "Lipochromes" section) (Figure 2, D and H).
(47) We now appreciate that colors in living organisms are the result of complex biochemical reactions with the production of biologic pigments (cytochromes, porphyrins, melanins, lipochromes) or because of structural coloration (iridescence).
prosthetic group) Iron Brown-black, bound to hemosiderin or ferritin, ferric state (Fe+3) Lipofuscin and Brown-black other lipochromes Autofluorescence Known as "wear and tear" pigment Melanins Eumelanin Brown-black Pheomelanin Golden yellow-red Neuromelanin Brown-black Intrinsic Unknown neuronal pigment?
However, through a complex series of HPLC studies of the red feathers in 44 species of parrots (of more than 350 total species, 80% of which have red plumage), McGraw and co-researcher Mary Nogare found a unique set of five molecules responsible for the red coloring in the feathers, called polyneal lipochromes, or psittacofulvins.
And the lipochromes that he found in the feathers were not found anywhere else in the parrot bodies, implying that they were being manufactured directly at the maturing follicles of the growing plumage.
They found that all the birds' red feathers contained a set of five pigments called polyenal lipochromes.
Unlike carotenoids, which birds obtain from their diets, lipochromes appear to be synthesized within the follicles of maturing parrot feathers, says McGraw.